In a water-cooled internal combustion engine such as a water-cooled engine for automobiles, a water jacket serving as a cooling water-circulating channel is formed around a bore wall of a cylinder bore. Normally, by inserting and assembling a spacer (water jacket spacer) into the inside of such a water jacket, flow of cooling water is controlled.
Such a water jacket spacer is generally produced by injection molding by using a prescribed resin material. Patent Document 1 proposes a method in which, in order to prevent deformation of a water jacket spacer in a post-molding cooling step without using a jig for correcting the shape, the opposing parts of the cylindrical main body of the spacer to be inserted into a water jacket are connected by a connecting bride, the shape of the spacer is retained after molding, and then the connecting bridge is cut off.